Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors (OFETs) [see H. E. Katz, Z. Bao and S. L. Gilat, Acc. Chem. Res., 2001, 34, 5, 359]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
There is a need for an improved method of preparing polymers, particularly poly-(3-substituted) thiophenes with high regioregularity, high molecular weight, high purity and high yields in an economical, effective and environmentally beneficial way, which is especially suitable for industrial large scale production.
It was an aim of the present invention to provide an improved process for preparing polymers with these advantages, but not having the drawbacks of prior art methods mentioned above.
Other aims of the present invention are immediately evident to the person skilled in the art from the following detailed description.
The inventors of the present invention have found that these aims can be achieved, and the above problems be solved, by providing a process of preparing polymers, in particular poly-(3-substituted) thiophenes according to the present invention as described below. According to this process, a 3-substituted thiophene monomer with at least two groups, wherein these groups are leaving groups that are capable of reacting with magnesium, is reacted with magnesium in a suitable solvent to form an intermediate, which is then polymerized in the presence of a suitable catalyst. It was surprisingly found that, by chosing appropriate reagents and reaction conditions, it is possible to obtain polymers, in particular poly-(3-substituted) thiophenes, with high regioregularity, high molecular weight and high purity in good yields and avoiding large amounts of hazardous by-products that need to be eliminated.
The polymers prepared by the process according to the present invention are especially useful as charge transport materials for semiconductor or light-emitting materials, components or devices. The above described routes of preparing polymers usually give a polymer that is terminated by reactive groups such as halogens, organometallic species such as boronate esters or trialkyl tins, or very reactive groups such as organo-magnesium or organo-zincs, which will be quenched under standard work up procedures. However, these groups have a disadvantageous effect on the electrical properties of the polymer; for example in semiconducting applications they can trap charges and thus reduce the charge carrier mobility of the polymer. It is therefore a further aim of this invention to further improve the electrical properties of the polymers obtained by the inventive process and also by the prior art methods as described above. The inventors have found that this aim can be achieved by chemical modification (‘endcapping’) of the polymers as described above and below. Thus, replacing the end groups of the polymers for example with alkyl groups or hydrogen can give poly(3-alkylthiophenes) with improved electrical properties. Another aspect of the invention therefore relates to a process of endcapping polymers, in particular regioregular poly (3-alkyl thiophenes) and to polymers obtained by this process.